I might have been blessed with an exceptional and rare situation but it happened to me... so it does exist. In Nov.2012 after lots of discussion and ground rules a friend and I began a fwb. We are truly friends but there is no way that we could ever be more than that for many reasons ( age difference, life-stage difference etc). I care for him and it is reciprocal. Neither of us were emotionally ready to trust in a romantic relationship but I did trust my friend.
One rule was that if either of us met someone we wanted to date then the benefits stop. At one point we took a break for a few months when his life became complicated and it did not interfere with the friendship at all. There were no hurt feelings.
I recently met someone that I was interested in and let him know. His response was that "the guy had better be good to you". 2 weeks later I saw him and he told me he had actually asked someone out! Strangely enough I am so happy for him!

I think we helped each other heal.
My situation did not work out but oh well. That guy's loss. Still happy for my friend. Do not regret a thing.